A. Optical Edge Filters and their Use
Optical edge filters are key components in important systems for optical measurement and analysis including Raman spectroscopy and fluorescence microscopy. Optical edge filters are used in these systems to block unwanted light that would otherwise generate spurious optical signals and swamp the signals to be detected.
Optical edge filters block unwanted light having wavelengths above or, alternatively, below a chosen “transition” wavelength λT while transmitting light on the unblocked side of λT. Edge filters which transmit optical wavelengths longer than λT are called long-wave-pass filters (LWP filters), and edge filters which transmit wavelengths shorter than λT are short-wave-pass or SWP filters.
Referring to the drawings, FIGS. 1A and 1B schematically illustrate the spectral transmission of idealized long-wave-pass and short-wave-pass filters respectively. As can be seen from FIG. 1A, a LWP filter blocks light with wavelengths below λT and transmits light with wavelengths above λT. As shown in FIG. 1B, a SWP filter transmits light with wavelengths below λT and blocks light with wavelengths above λT. λT is the wavelength at which the filter “transitions” from blocking to transmission, or vice versa.
While an ideal edge filter has a precise transition wavelength λT represented by a vertical line at λT, real edge filters change from blocking to transmission over a small range of wavelengths and are more accurately represented by a non-vertical but steeply sloped line near λT. Similarly, while an ideal edge filter transmits all light in the transmission region (transmission T=1), real filters invariably block a small portion of the light to be transmitted (T<1). The steepness of the line and the proportion of the light transmitted are important parameters in many applications.
Edge filters are particularly useful in optical measurement and analysis systems that use light of one wavelength to excite a sample and measure or view an optical response of the excited sample at other wavelengths. The excitation light is delivered to the sample by an excitation light path, and the optical response of the sample is delivered to the eye or measuring instrument by a collection path. Edge filters can be used to block spurious light from the excitation path and to block excitation light from entry into the collection path. The steeper the filter edge, the more effectively spurious signals are blocked. The lower the transmission loss, the more light from the sample reaches the measuring instrument.
Raman spectroscopy is one such optical analysis system. It is based on the fact that when molecular material is irradiated with high intensity light of a given wavelength λ, a small portion of the incident light scattered by the material will be shifted in wavelength above and below λ. This Raman shifting is attributed to the interaction of the light with resonant molecular structures within the material, and the spectral distribution of the Raman-shifted light provides a spectral “fingerprint” characteristic of the composition of the material. As a practical example, one can use a Raman probe to identify the contents of a bottle without opening the bottle.
FIG. 2 is a simplified schematic diagram of a Raman probe 20 designed to excite and collect the long wavelength portion of Raman-shifted light from a sample 21. In essence, the probe 20 comprises an optical fiber excitation path 22, and a fiber collection path 23. Edge filters 22A and 23A are disposed in the respective paths.
In operation, excitation light from a laser 24 passes through the fiber path 22 and edge filter 22A to illuminate a portion of the sample 21 with high intensity light of a wavelength λ. Light scattered from the sample 21 passes through edge filter 23A and then through fiber collection path 23 to a spectral analyzer 25 where the “fingerprint” of the sample is determined.
Since the fiber 22 through which the excitation signal passes is composed of molecular material, a small portion of the excitation light will be shifted in wavelength by the Raman effect in the fiber. This shifted light must be eliminated to prevent false readings. The removal can be accomplished by disposing a SWP edge filter 22A between the fiber 22 and sample 21. SWP edge filter 22A, having a transition wavelength just above the laser wavelength, blocks both long wavelength Raman scattering from the fiber and long wavelength noise from the laser.
However, in some cases, a filter 22A is not needed. For instance, if the excitation signal is direct laser radiation that does not pass through molecular material at any appreciable length, the filter 22A is not necessary. An example would be direct laser radiation illuminating a sample 21 through a vacuum.
The light scattered from the sample 21 is a mixture of unshifted scattered excitation light (Rayleigh scattering) and Raman-shifted light. The scattered excitation light would not only swamp the analyzer, it would also excite spurious Raman scattering in the collection fiber. Thus the unshifted excitation light should be removed from the collection path. This can be accomplished by disposing a long pass edge filter 23A between the sample 21 and the collection fiber 23, the long pass filter having a transition wavelength λT just below the excitation wavelength λ. This arrangement ensures that the light reaching the analyzer is predominantly the long wavelength Raman-shifted light from the sample. Analogous arrangements using edge filters can be used to analyze short wavelength Raman-shifted light.
Edge filters are equally useful in fluorescence microscopy. Here excitation light is used to excite longer wavelength emission from fluorescent markers. The markers can be fluorescent atoms chemically bonded to a biological molecule to track the molecule in a body or cell. Edge filters are used, as in Raman spectroscopy, to reject spurious low wavelength light from the excitation path and to reject excitation light from the collection path.
It should now be clear that the steeper the filter slope at the transition wavelength λT the greater the amount of spurious light that can be filtered out. In addition, the steeper the slope, the greater the amount of shifted light from the sample that will reach the analyzer. Similarly, higher levels of transmission of the shifted light through the filters provide more light for analysis. Higher edge filter blocking provides better rejection of the laser excitation light from the spectrum analyzer, thus decreasing the noise and improving both specificity and sensitivity of the measurement. Higher edge-filter transmission enables the maximum signal to reach the analyzer, further improving the signal-to-noise ratio and hence the measurement or image fidelity. A steeper filter edge also permits shifts to be resolved much closer to the excitation wavelength, thus increasing the amount of information from the measurement.
B. Edge Filter Structure and Conventional Fabrication
FIG. 3 is a simplified schematic illustration of an optical edge filter 30 comprising a transparent substrate 31 having a flat major surface 32 supporting many thin coatings 33A, 33B. The thickness of the coatings is exaggerated and the number is reduced for purposes of illustration. Coatings 33A and 33B are typically alternating and of different respective materials chosen to present markedly different indices of refraction (index contrast). The coating indices and thicknesses are chosen and dimensioned to filter impinging light by interference effects in a desired manner. Specifically, if a light beam 34 impinges on the filter, a first wavelength portion 34T of the beam is transmitted and a second wavelength portion 34R is reflected and thus rejected by the filter. What is transmitted and what is reflected depends on the precise thickness and indices of the thin coatings. There are two basic types of thin-film edge filters: those based on “soft coatings” and those based on “hard coatings,” both of which are typically manufactured by an evaporation technique (either thermal evaporation or electron-beam evaporation). Hard coating filters, however, may also be manufactured by non-evaporative techniques such as ion beam sputtering.
Soft coatings imply literally what the name suggests—they are physically soft and can be readily scratched or damaged. They are fairly porous, which also means they tend to be hygroscopic (absorb water vapor) leading to dynamic changes in the film index and hence the resulting filter spectrum in correlation to local humidity. There are two main reasons soft coatings are used. First, an advantageous larger index contrast can be realized with soft coatings. (The index contrast is the relative difference between the index of refraction of the low-index material and that of the high-index material.) For example, many high-performance soft-coated filters are made using sodium aluminum fluoride (“cryolite”), with a chemical composition of Na3AlF6 and an index of about 1.35 for visible wavelengths, and zinc sulfide, with a chemical composition of ZnS and an index of about 2.35. The second reason for using these materials is that the evaporation process can be controlled well for these materials, largely because they have relatively low melting temperatures. Hence it is possible to maintain fairly accurate control over the layer thicknesses even for filter structures with many 10's of layers and perhaps even up to 100 layers. As described above, edge filter performance is measured by edge steepness, depth of blocking, and high transmission with low ripple. A larger index contrast and a larger number of layers both yield more steepness and more blocking. High transmission with low ripple is improved with more layers and higher layer thickness accuracy. For these reasons the highest performance conventional thin-film edge filters have been made with soft-coating technology.
Hard coatings are made with tougher materials (generally oxides), and result from “energetic” deposition processes, in which energy is explicitly supplied to the film itself during the deposition process. This is accomplished with a beam of ions impinging directly on the coating surface. The ion bombardment acts to “hammer” the atoms into place in a more dense, less porous film structure. Such processes are usually called ion-assisted deposition (IAD) processes. High-performance edge filters have been made with ion-assisted electron-beam evaporation. Typically the index contrast available with hard-coating (oxide) thin-film materials is not as high as that of the soft-coating materials, and consequently more layers must be deposited to achieve a comparable level of performance. This problem, coupled with the more difficult to control deposition rates and overall processes of high-melting-temperature oxides, leads to much more stringent requirements on the layer-thickness control techniques to achieve a reasonable level of layer thickness accuracy for good edge steepness and high, low-ripple transmission.
For the best edge filters, some kind of “optical monitoring” (direct measurement of filter transmission or reflection during deposition) is necessary to determine when to terminate the deposition of each layer. Optical monitoring can be performed on the actual filters of interest or on “witness pieces” often positioned in the center of the deposition chamber. There are three basic types of optical monitoring algorithms. The first is often called “drop-chip” monitoring, and is based on measuring the transmission (or reflection) vs. time through a new witness piece for each new layer. Since the theoretical transmission vs. time can be calculated accurately for each layer deposited on a blank piece of glass, then a good comparison between the measured and theory curves can be made independent of the history of the deposition (thickness errors in previous layers). This technique is accurate and useful for layers of arbitrary thickness, but it is cumbersome, especially for filters comprised of at least many 10's of layers.
The second type of monitoring is called “turning-point” monitoring, and is used for depositing layers that are precisely a quarter of a wavelength in thickness (or multiples thereof). The technique is based on the fact that the transmission vs. time reaches a turning point (or extremum) at each multiple of a quarter wave of thickness, so an algorithm is developed to cut layers precisely at the turning points. The elegant feature of this method is that there is inherent compensation for layer thickness errors from previous layers, so long as one adheres to the rule of cutting exactly at turning points. It thus works extremely well even for very thick coatings with even hundreds layers (it is the basis for manufacturing very high-performance filters for DWDM telecom applications, which can have as many as 200–400 quarter-wave layers).
The third type of monitoring is called “level monitoring,” and is applicable for non-quarter-wave thick layers. Monitoring can be done through the actual filters or through witness piece(s). The concept is to cut layers at predetermined transmission levels, based on a calculated prediction of transmission vs. time for the entire structure. However, because small layer errors lead to large variations in the absolute transmission values, one must instead rely on cutting at the correct transmission level relative to the local maximum and minimum values. Hence the method works well only for non-quarter-wave thick layers that are more than a half-wave thick, so that there is both a maximum and a minimum transmission value in the transmission vs. time curve for that layer. Even in this case, this method does not contain inherent compensation for errors in the thickness of previously deposited layers, and thus is not as forgiving as the turning-point method. However, to obtain an edge filter with high transmission and low ripple requires primarily non-quarter-wave thick layers, and hence turning-point monitoring is not applicable for edge filters.
Accordingly there is a need for an improved method of making an optical edge filter and for improved edge filters having increased edge steepness and reduced transmission.